NADPH oxidases (NOX) are proteins that transfer electrons across biological membranes. In general, the electron acceptor is oxygen and the product of the electron transfer reaction is superoxide. The biological function of NOX enzymes is therefore the generation of reactive oxygen species (ROS) from oxygen. Reactive oxygen species (ROS) are oxygen-derived small molecules, including oxygen radicals (super-oxide anion [●O2−], hydroxyl [HO●], peroxyl [ROO●], alkoxyl [RO●.] and hydroperoxyl [HOO●]) and certain non-radicals that are either oxidizing agents and/or are easily converted into radicals. Nitrogen-containing oxidizing agents, such as nitric oxide are also called reactive nitrogen species (RNS). ROS generation is generally a cascade of reactions that starts with the production of superoxide. Superoxide rapidly dismutates to hydrogen peroxide either spontaneously, particularly at low pH or catalyzed by superoxide dismutase. Other elements in the cascade of ROS generation include the reaction of superoxide with nitric oxide to form peroxynitrite, the peroxidase-catalyzed formation of hypochlorous acid from hydrogen peroxide, and the iron-catalyzed Fenton reaction leading to the generation of hydroxyl radical.
ROS avidly interact with a large number of molecules including other small inorganic molecules as well as DNA, proteins, lipids, carbohydrates and nucleic acids. This initial reaction may generate a second radical, thus multiplying the potential damage. ROS are involved not only in cellular damage and killing of pathogens, but also in a large number of reversible regulatory processes in virtually all cells and tissues. However, despite the importance of ROS in the regulation of fundamental physiological processes, ROS production can also irreversibly destroy or alter the function of the target molecule. Consequently, ROS have been increasingly identified as major contributors to damage in biological organisms, so-called “oxidative stress”.
During inflammation, NADPH oxidase is one of the most important sources of ROS production in vascular cells under inflammatory conditions (Thabut et al., 2002, J. Biol. Chem., 277:22814-22821).
In the lung, tissues are constantly exposed to oxidants that are generated either endogenously by metabolic reactions (e.g. by mitochondrial respiration or activation of recruited inflammatory cells) or exogenously in the air (e.g. cigarette smoke or air pollutants). Further, the lungs, constantly exposed to high oxygen tensions as compared to other tissues, have a considerable surface area and blood supply and are particularly susceptible to injury mediated by ROS (Brigham, 1986, Chest, 89(6): 859-863). NADPH oxidase-dependent ROS generation has been described in pulmonary endothelial cells and smooth muscle cells. NADPH oxidase activation in response to stimuli has been thought to be involved in the development of respiratory disorders such as pulmonary hypertension and enhancement of pulmonary vasoconstriction (Djordjevic et al., 2005, Arterioscler. Thromb. Vasc. Biol., 25, 519-525; Liua et al., 2004, Am. J. Physiol. Lung, Cell. Mol. Physiol., 287: L111-118). Further, pulmonary fibrosis has been characterized by lung inflammation and excessive generation of ROS.
Osteoclasts, which are macrophage-like cells that play a crucial role in bone turn-over (e.g. bone resorption), generate ROS through NADPH oxidase-dependent mechanisms (Yang et al., 2002, J. Cell. Chem. 84, 645-654).
Diabetes is known to increase oxidative stress (e.g. increased generation of ROS by auto-oxidation of glucose) both in humans and animals and increased oxidative stress has been said to play an important role in the development of diabetic complications. It has been shown that increased peroxide localization and endothelial cell dysfunction in the central retina of diabetic rats coincides with the areas of NADPH oxidase activity in the retinal endothelial cells (Ellis et al., 2000, Free Rad. Biol. Med., 28:91-101). Further, it has been suggested that controlling oxidative stress (ROS) in mitochondria and/or inflammation may be a beneficial approach for the treatment of diabetes (Pillarisetti et al., 2004, Expert Opin. Ther. Targets, 8(5):401-408).
ROS are also strongly implicated in the pathogenesis of atherosclerosis, cell proliferation, hypertension and reperfusion injury cardiovascular diseases in general (Cai et al., 2003, Trends Pharmacol. Sci., 24:471-478). Not only is superoxide production, for example in the arterial wall, increased by all risk factors for atherosclerosis, but ROS also induce many “proatherogenic” in vitro cellular responses. An important consequence of the formation of ROS in vascular cells is the consumption of nitric oxide (NO). NO inhibits the development of vascular diseases, and loss of NO is important in the pathogenesis of cardiovascular diseases. The increase in NADPH oxidase activity in vascular wall after balloon injury has been reported (Shi et al., 2001, Throm. Vasc. Biol., 2001, 21, 739-745).
It is believed that oxidative stress or free radical damage is also a major causative factor in neurodegenerative diseases. Such damages may include mitochondrial abnormalities, neuronal demyelination, apoptosis, neuronal death and reduced cognitive performance, potentially leading to the development of progressive neurodegenerative disorders (Nunomura et al., 2001, J. Neuropathol. Exp. Neurol., 60:759-767; Girouard, 2006, J. Appl. Physiol. 100:328-335).
Further, the generation of ROS by sperm has been demonstrated in a large number of species and has been suggested to be attributed to an NADPH oxidase within spermatozoa (Vernet et al., Biol. Reprod., 2001, 65:1102-1113). Excessive ROS generation has been suggested to be implicated in sperm pathology, including male infertility and also in some penile disorders and prostate cancer.
NADPH oxidases are multi-subunit enzymes made up of a membrane-bound cytochrome b558 domain and three cytosolic protein subunits, p47phox, p67phox and a small GTPase, Rac. Seven isoforms of NOX enzymes have been identified including NOX1, NOX2, NOX3, NOX4, NOX5, DUOX1 and DUOX2 (Leto et al., 2006, Antioxid Redox Signal, 8(9-10):1549-61; Cheng et al., 2001, Gene, 16; 269(1-2):131-40).
Thus, ROS derived from NADPH contribute to the pathogenesis of numerous diseases, especially cardiovascular diseases or disorders, respiratory disorder or disease, disease or disorder affecting the metabolism, bone disorders, neurodegenerative diseases, inflammatory diseases, reproduction disorder or disease, pain, cancer and disease or disorders of the gastrointestinal system. Therefore, it would be highly desirable to develop new active agents focusing on the ROS signalling cascade, especially on NADPH oxidases (NOX).